1. Field of the Invention
The present invention relates to a driving circuit for organic thin film EL elements which utilizes an electro luminescence (EL) phenomenon of organic thin films, and more specifically a driving circuit for organic thin EL films which is to be used for displaying characters and figures by driving a matrix of EL elements.
2. Description of the Prior Art
There is known a fact that when a certain organic thin film which is interposed between an anode and a cathode is electrically energized, positive holes and electrons poured from the respective electrodes recombine with each other in the organic film, whereby a luminescent phenomenon takes place due to energies produced by the recombination. This phenomenon is referred to as an organic thin film EL. Since an organic thin film EL element has merit that it can be driven with a DC voltage on the order of several to ten-odd volts, emits rays at a higher efficiency, and is thinner and lighter in weight than other display devices, researches are now being made vigorously for application to various kinds of light-emitting devices.
Though the EL phenomenon can take place even when an organic thin film which is capable of transmitting light (hereinafter referred to as an organic light-emitting thin film layer) is composed of a single layer, it is necessary for obtaining high luminance at a low voltage to pour a carrier from each electrode into the organic light-emitting thin film layer with an enhanced efficiency. Accordingly, there have been proposed laminated structures wherein additional carrier pouring layers or carrier transport layers are interposed between electrodes and organic light-emitting thin film layer for lowering energy barriers between the electrodes and the organic light-emitting thin film layers, thereby facilitating to shift carriers into the organic light-emitting thin film layers. For example, Japanese Patent Application Laid-Open No. 57-51781 proposes a structure which is composed of an anode/an organic positive hole transport layer/an organic light-emitting thin film layer/a cathode and Japanese Patent Application Laid-Open No. 6-314594 proposes a structure which is composed of an anode/a plurality of organic positive hole pouring transport layer/an organic light-emitting thin film layer/a plurality of organic electron pouring transport layer/a cathode. The laminating sequence may be reversed. FIG. 5 shows a sectional view of an organic thin film EL element having a general laminated structure which is composed of an anode/an organic positive hole transport layer/a light-emitting thin film layer/a cathode formed on a support substrate, and means for applying a voltage to this element.
Materials which are used for composing the organic thin film EL element will be described with reference to FIG. 5. Speaking of electrodes first, at least one of the cathode and anode must be transparent since light must be taken out of the organic light-emitting thin film layer. In most cases, a thin film of indium-tin oxide (ITO) or a thin film of gold is used as an anode 31. On the other hand, a material which has a small work function is selected for a cathode 34 for the purpose of lowering a pouring barrier to electrons and a film of a metal such as magnesium, aluminium, indium or an alloy thereof is used as the cathode 34. Aromatic amine class 3, a polyphyrine derivative or the like is used as an organic positive hole transport layer 32 and 8-hydroxyquinoline metal complex, a butadiene derivative, a benzoxadole derivative or the like is used as an organic light-emitting thin layer 33. In case of a structure which has an organic electron transport layer, a naphthalimide derivative, a perylene tetracarbonate di-imide derivative, quinacridon derivative or the like is additionally used though the organic thin film EL element shown in FIG. 5 does not use such a substance. The electrodes and the organic thin film layers are formed on a support substrate made of a glass or resin material by a dry film forming method such as vacuum deposition or sputtering or by a wet film forming method such as spin coating or dipping by gradually laminating the material mentioned above from a solution in which the material mentioned above is dissolved or dispersed. When a transparent electrode (the anode 31 in this case) is formed as a first layer, a support substrate 30 must also be made of a transparent substance.
When a voltage is applied to an EL element which is composed as described above, it exhibits a voltage-current characteristic like that of a diode as shown in FIG. 6. It is therefore general to drive the element with a current.
As devices to which organic thin film EL elements having structures and electric characteristics like those described above are applied, there have conventionally been proposed planar surface light-transmitting type organic thin film EL displays which drive matrices of organic thin film EL elements exemplified above as unit picture elements arranged in two dimensions on planar surfaces of support substrates. Japanese Patent Application Laid-Open No. 7-36410 discloses an example (conventional example 1) of such a device. Referring to FIG. 7 which illustrates a theoretical circuit of a driving circuit of a conventional example 1 proposed by this Japanese patent, a display panel 10 is driven by an X driver 12 and a Y driver 14. A matrix of the display panel 10 is composed of signal electrodes 16-0, 16-1, 16-2, . . . from the X driver 12 and scanning electrodes 18-0, 18-1, . . . from the Y driver 14. A light-emitting element 20 is connected to each intersection of the matrix. The X driver 12 comprises constant-voltage power sources 22-0, 22-1, 22-2, . . . which receive a driving pulse signal 26 together with a power source voltage (=+V) from a control computer 24 and output a constant current for igniting the light-emitting elements to the signal electrodes 16-0, 16-1, 16-2, . . . . Further, the Y driver 14 comprises switch elements 28-0, 28-1, . . . which are turned on and off by a control signal 29 from the control computer 24 to connect and disconnect the scanning electrodes 18-0, 18-1, . . . to and from ground, thereby driving a matrix.
FIG. 11 illustrates a more concrete composition of the circuit shown in FIG. 7 described above.
In FIG. 11, a video signal is supplied to a shift register 38 used as a memory by way of an A/D converter 36 which comprises a plurality of flip-flop circuits (hereafter referred to as FFs) 44 through 44. Signals from the FFs in the shift register 38 are supplied to PWM modulators 48 through 48 by way of FFs 46 through 46 in an X driver 40. Signals (analog signals indicating pulse widths corresponding to luminance data) from the PWM modulators 48 through 48 are supplied to signal electrodes A0, A1, A2, A3, . . . , whereas signals from FFs 50 through 50 in a Y driver 34 are supplied to scanning electrodes K0, K1, K2, K3, . . . , whereby a matrix of a display panel 30 is composed of the signal electrodes A0, A1, A2, A3, . . . and the scanning electrodes K0, K1, K2, K3, . . . . Light emitting elements 52 through 52 are connected to the signal electrodes A0, A1, A2, A3, . . . and the scanning electrodes K0, K1, K2, K3, . . . at intersections between the signal electrodes A0, A1, A2, A3, . . . and the scanning electrodes K0, K1, K2, K3, . . . .
A timing generator 42 which is used as a controller receives a horizontal synchronizing signal and a vertical synchronizing signal, and outputs signals SCLK, LCLK, FPUL and FCLK. The signal SCLK is supplied to the A/D converter 36 and the FFs 44 through 44 in the shift register 38, the signal LCLK is supplied to the FFS 46 through 46 in the X driver 40, and the signals FPUL and FCLK are supplied to the FFs 50 through 50 in the Y driver 34.
Describing with reference to a timing chart of the X driver shown in FIG. 12(A), data DATA which has been subjected to A/D conversion is shifted sequentially to the FFS 44 through 44 in the shift register 38 by the signal SCLK each time the video signal is subjected to A/D conversion and sampled. When all the data DATA in a single horizontal synchronizing period is sent to the FFs 44 through 44, data in the FFs 44 through 44 is supplied by the signal LCLK to the PWM modulators 48 through 48 by way of the FFs 46 through 46 in the X driver 32. The PWM modulators 48 through 48 perform PWM modulation of the sent data and output pulses having lengths corresponding to the data to the signal electrodes A0, A1, A2, A3, . . . .
Describing with reference to a timing chart of the Y driver shown in FIG. 12(B), the signal FPUL is set at a "High" level once during a vertical synchronizing period and a pulse of the signal FPUL is transmitted by the signal FCLK sequentially to the scanning electrodes (lines) K0, K1, K2, K3 . . . When a scanning line Kn (n=0, 1, 2, 3, . . . ) is ignited when it is set at the "High" level. The signal FCLK outputs a pulse during one horizontal synchronizing period and the signal FPUL outputs a pulse during one vertical synchronizing period.
Japanese Patent Application Laid-Open No. 7-36410 mentioned as the conventional example 1 discloses a method which drives light-emitting elements arranged in a shape of a matrix with a constant current as described above.
Further, Japanese Patent Application Laid-Open No. 3-157690 discloses a second method (conventional example 2) which is conventionally used for driving a thin film EL display. It is a driving method for displaying gradations by applying a pulse width modulation system to a display unit EL in which EL elements are interposed between a plurality of scanning side electrodes and a plurality of data side electrodes arranged in directions intersecting with each other, and configured to drive a thin film EL display by using, as a voltage to be applied to each picture element on selective scanning electrodes, a pulse voltage having waveform in which a crest at a front portion of a pulse is higher than that at a rear portion of the pulse. Referring to FIG. 8 which shows the pulse waveform obtained by the conventional example 2, a pulse waveform in a light-emitting condition at maximum luminescence B max is illustrated in FIG. 8(a), a pulse waveform in a light-emitting condition at medium luminescence BX is illustrated in FIG. 8(b), and a pulse waveform in a non-light-emitting condition (luminescence B0) is illustrated in FIG. 8(c). This method uses a lamp voltage having a waveform which lowers a crest from the front portion of the pulse to the rear portion of the pulse. The driving method according to the conventional example 2 is used mainly for driving an EL display which has a first field and a second field and, is driven with an AC voltage. This method is configured to cancel electric charges accumulated in light-emitting layers composing picture elements by applying a high voltage (Vw) at an initial light-emitting stage for displaying gradations free from luminance ununiformities when EL elements are operated with an effective voltage (Vw2) in the vicinity of a threshold value for light emission free from influences due to accumulated electric charges. The conventional reference 2 is an invention which relates to a method for driving the EL elements with an AC voltage.
A first problem proposed by the prior art described above is that luminance is not enhanced due to retardation in rise of pulses when the EL elements are driven with a square pulse signal in the planar surface light-emitting type organic thin film EL display according to the conventional example 1 in which the constant-current driving signals are supplied to the signal electrodes dependently on input signals. Since the organic thin film EL elements have a junction capacity, the capacity is charged first upon driving with the constant current, whereby a certain time is required until a voltage is enhanced to a level at which a light-emitting operation starts.
Extracting only a portion of the circuit diagram shown in FIG. 7 which corresponds to a single picture element for simplicity of description or facilitating understanding, the conventional example 1 drives an organic thin film EL element 20 with a circuit illustrated in FIG. 9. When the organic EL element 2 is driven with a square pulse signal 26, a pulse voltage indicated by OAPQ of a voltage waveform shown in FIG. 10 is applied to the EL element 20. In FIG. 10, a voltage VF along the ordinate is a forward voltage of the EL element and a voltage Va is a voltage at which the EL element starts emitting light. A time ta along the abscissa is a time as measured from a start of driving with the pulse to a start of the light emission. Further, a time T is a duration of time during which the driving pulse is applied to the EL element, or approximately 104 .mu.s when the EL element is driven for dynamic ignition at 1/64 duty and a repetition frequency of 150 Hz.
Referring to FIG. 10, it will be understood that the EL element emits light actually for a time of (T-ta) though the driving pulse is originally applied to the EL element for the time T and that luminance of the emission is lowered at a degree corresponding to the time ta. Speaking of a concrete example, a junction capacity is approximately 670 pF and the time ta is approximately 30 .mu.s when the EL element has a size of 0.52 mm.times.0.52 mm. The time ta=30 .mu.s is not negligible as compared with the time T=104 .mu.s. Since peak luminance lies at 13800 cd/m.sup.2 (at a DC current), mean luminance is remarkably lowered to 126 cd/m.sup.2 though it should originally be 216 cd/mm.sup.2. When a matrix has a larger scale and a duty is reduced, the time T is shortened with the time ta kept unchanged. At ta&gt;T, the EL element cannot emit light.
Then, the prior art poses a second problem that the planar surface light emitting type thin film EL display according to the conventional example 1 shortens a service lives of the EL elements. Luminance of the EL elements is determined dependently on current levels. Therefore, it is necessary to set a current level higher than required or supply a current in a larger amount to the EL elements in order to obtain required luminance without correcting the slow rise of the driving pulse described above. As a result, heating of the EL elements accelerates deterioration of these elements.